The present invention generally is directed to asychronous data transmission in switching systems. More specifically, the invention is directed to a method for monitoring the transmission bit rate defined for asychronous transmissions of message cells during the course of the set-up of virtual connections on transmission lines in switching systems, particularly broad band switching systems, wherein message cells whose transmission would exceed the transmission bit rate defined for the relevant connection are discarded.
It is known to define a maximum transmission bit rate in asychronous transmission switching systems during the course of set-up of virtual connections between, among other things, a subscriber station and a broadband switching node. Such a transmission bit rate, for example, can be established by a defined plurality of message cells having constant lengths per unit of time. Monitoring devices, generally referred to as policing equipment, are provided at the access locations of the switching network for monitoring the defined transmission bit rate.
It is known from an article by J. S. Turner, titled "New Directions in Communications" and published in the International Zurich Seminar on Digital Communications, March 1986, at A3.1 through A3.8, to allocate a forward-backward counter in a switching node of a broadband switching system to every subscriber station connected thereto. These forward-backward counters count the message cells transmitted by the respective subscriber stations and decrement a momentary counter reading at defined times in accordance with the transmission bit rate defined by the respective subscriber station. When the momentary counter reading upwardly exceeds a value prescribable by the respective subscriber station, then the switching node recognizes an overload condition. This can be referred to as the "leaky bucket" method.
When accessing an offering trunk shared in common by a plurality of subscriber stations that, for example, connects a private branch exchange (to which the subscriber stations are connected) to the broadband switching system, temporary transgressions of the defined transmission bit rate can occur at the access location of the broadband switching system despite adherence to the defined transmission bit rate by the subscriber station terminal equipment. An increase of the subscriber station associated transmission bit rate at the access location of the broadband switching system can result when two subscriber stations wish to simultaneously access the common offering trunk and thereby cause an access conflict that is resolved by delaying the transmission of individual message cells, so that the message cell following a delayed message cell in a virtual connection thereby affected appears within a shorter time frame than would correspond to the transmission bit rate declared during the call set-up and observed by the pertinent subscriber station terminal equipment.
Connection associated policing equipment arranged at the access location of the broadband switching system that functions according to the traditional leaky bucket method must tolerate temporary, upward transgressions of brief duration of the respectively declared transmission bit rates that can derive from jitter resulting on the offering trunk. Such tolerant behavior by traditional policing equipment vis-a-vis brief duration transgressions of the connection associated declared transmission bit rates, however, could be exploited in such a way that at least a limited number of message cells are transmitted by a subscriber station terminal equipment in a more rapid succession than corresponds to the declared transmission bit rate without the policing equipment at the input of the broadband switching system responding. As a result, the performance capability of a broadband switching system can deteriorate.